The invention relates generally to the field of digital imaging, and more particularly to an apparatus and method for reducing smear in digital images captured using a frame-transfer charge coupled device (CCD) sensor.
In contrast to a conventional film camera, a digital camera employs an electronic image sensor to digitally capture a scene of interest. Thus, the electronic image sensor functions as a film for the digital camera. Typically, the electronic image sensor is either a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor. However, due to their higher sensitivity and lower noise characteristics, CCD sensors have been much more widely adopted in applications that require high image quality.
There are a number of different types of CCD sensors, such as frame interline-transfer CCD sensors, interline-transfer CCD sensors, and frame-transfer CCD sensors. The CCD sensors of interest herein are the frame-transfer CCD sensors. A conventional frame-transfer CCD sensor for use in digital photography includes a CCD array of pixel regions and a serial register. The CCD array includes an image area and a storage area. The pixel regions of the CCD array are defined by isolation implants and gate electrodes, which extend perpendicular to each other. The CCD array is used to accumulate electronic charges within the pixel regions in response to impinging illumination and to transfer the accumulated charges to the serial register for readout. However, the storage area of the CCD array is covered with an opaque mask to prevent photon-induced accumulation of charges. Thus, the storage area is exclusively used to transfer the accumulated charges in the image area to the serial register for readout. The storage area allows the accumulated charges in the image area to be rapidly removed from exposure to illumination.
A digital camera with a conventional frame-transfer CCD sensor digitally captures images in the following manner. First, the image area of the CCD array is cleared of existing charges, which begins an exposure period. During the exposure period, charges are accumulated in the pixel regions of the image area in response to impinging light. At the end of the exposure period, the accumulated charges in the image area are rapidly transferred to the storage area of the CCD array. The transferred charges are then readout through the serial register on a row-by-row basis in a relatively slow fashion.
Typically, a digital camera operates in either a still-capture mode or a video mode. The still-capture mode is used to capture still images of scenes of interest. The video mode is used to acquire information for exposure control and focus control. In addition, the video mode is used to provide a user with a real time preview of the final image.
In the still-capture mode, a mechanical shutter of the digital camera is used to end the exposure period. The mechanical shutter prevents further accumulation of charges in the image area of the CCD array. However, in the video mode, the high frame rate makes the use of the mechanical shutter impractical. Thus, the mechanical shutter is not used during the video mode. As a result, photon-induced charges may continue to accumulate even after the exposure period when the accumulated charges are being transferred from the image area of the CCD array to the storage area of the CCD array. The additional accumulation of charges may result in the appearance of xe2x80x9csmearxe2x80x9d in the captured images. The relative magnitude of the xe2x80x9csmear chargexe2x80x9d is inversely proportional to the ratio of the exposure period to the period required to shift the accumulated charges by one row (the horizontal line period) to transfer the accumulated charges to the storage area of the CCD array. Smear is most noticeable when the exposure period is short and the illumination is strong. Smear can be reduced by increasing the speed of the frame shift rate, i.e., the vertical transfer rate of charges from the image area of the CCD array to the storage area of the CCD array. However, the frame shift rate is limited to about 1 MHz in current frame-transfer CCD sensors by the RC time-constant of the gate electrodes. Thus, the appearance of smear is still a significant issue for digital images captured using a conventional frame-transfer CCD sensor.
In view of the above-described problem, there is a need for an apparatus and method for reducing smear in electronically captured images using a frame-transfer CCD sensor.
An apparatus and method for reducing smear in electronic images utilizes estimated smear signals to remove components in the image signals of the electronic images that are attributable to the smear. The estimated smear signals correspond to the smear components of the image signals. The estimated smear signals are generated by collecting electrical charges in an electronic image sensor after an exposure period. The estimated smear signals may be generated in sequence to the acquisition of the image signals. Alternatively, the estimated smear signals may be generated in parallel to the acquisition of the image signals.
A method for reducing smear in images captured using an electronic image sensor in accordance with the invention includes the steps of generating image signals, some of which include image and smear components, and generating estimated smear signals, which are based on electrical charges accumulated in the electronic image sensor. The image signals represent an electronic image of a scene of interest. The image components of the image signals correspond to electrical charges collected in the electronic image sensor during an exposure period. The smear components of the image signals correspond to electrical charges collected in the electronic image sensor after the exposure period. The smear signals represent the smear components of the image signals. The method further includes the step of substantially removing the smear components from the image signals using the estimated smear signals to reduce smear in the electronic image.
The step of generating the image signals may include collecting electrical charges in the electronic image sensor from the start of an exposure period. Furthermore, the step of generating the estimated smear signals may include collecting electrical charges in the electronic image sensor from the end of an exposure period. In an embodiment, the step of generating the estimated smear signals and the step of generating the image signals are executed in sequence.
In another embodiment, the step of generating the estimated smear signals and the step of generating the image signals are executed in parallel. In this embodiment, the step of generating the estimated smear signals may include collecting electrical charges in selected portions of the electronic image sensor from the end of an exposure period to acquire sample smear signals. The selected portions of the electronic image sensor may be selected photosensitive pixel rows of the image sensor. In this embodiment, the method may further include a step of interpolating the sample smear signals from adjacent rows of the selected photosensitive pixel rows to derive the estimated smear signals.
An apparatus in accordance with the invention includes an electronic image sensor having an array of photosensitive pixels and a smear cancellation unit. The electronic image sensor is configured to generate image signals that represent an electronic image of a scene of interest. The electronic image sensor may include a charge coupled device. Some of the image signals generated by the electronic image sensor include image components and smear components. The image components of the image signals correspond to electrical charges collected in the electronic image sensor during an exposure period. The smear components of the image signals correspond to electrical charges collected in the electronic image sensor after the exposure period. The electronic image sensor is further configured to generate sample smear signals that represent the smear components of the image signals. The smear cancellation unit of the apparatus is configured to substantially remove the smear components from the image signals using the sample smear signals to reduce smear in the electronic image.
In an embodiment, the electronic image sensor is configured to generate a set of the image signals and a set of the sample smear signals in sequence. In this embodiment, the smear cancellation unit includes a subtraction unit that subtracts the image signals by the sample smear signals to substantially remove the smear components from the image signals.
In another embodiment, the electronic image sensor is configured to generate the image signals and the sample smear signals in parallel. In this embodiment, the electronic image sensor includes selected photosensitive pixels that are configured to exclusively generate the sample smear signals. The selected photosensitive pixels may be located on selected pixel rows of the electronic image sensor. In this embodiment, the smear cancellation unit may include an interpolation unit that generates estimated smear signals from the sample smear signals from adjacent pixel rows of the selected pixel rows of the electronic image sensor. The smear cancellation unit may also include a subtraction unit that subtracts the image signals associated with pixel rows of the electronic image sensor that are positioned between the adjacent pixel rows of the electronic sensor by the estimated smear signals to substantially remove the smear components from the image signals.
Other aspects of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.